Backside binary grated lens coupled to front side waveguide

ABSTRACT

A wafer structure includes a diffractive lens disposed on a backside of a wafer and coupled to a front side waveguide, the diffractive lens being configured to receive light and focus the light to the front side waveguide.

FIELD OF THE INVENTION

The invention relates to semiconductor structures and, moreparticularly, to an integrated circuit with a backside binary gratedlens coupled to a front side waveguide and a method of manufacture.

BACKGROUND

Transmitting light from a light source, such as a laser, across asemiconductor structure, such as an integrated circuit, can be difficultto achieve. For example, sophisticated alignment packaging schemesneeded to align the integrated circuit to an optical fiber. Thisalignment needs to be accurate to the submicron level, which can be verycostly. Also, the diameter of fiber, and a beam of light output by thefiber, can be substantially larger, e.g., by a factor of 200, than thediameter of a waveguide. Because of this large difference in diameter,substantial optical loss often occurs when coupling the fiber to thewaveguide.

SUMMARY

In an aspect of the invention, a wafer structure comprises a diffractivelens disposed on a backside of a wafer and coupled to a front sidewaveguide, the diffractive lens being configured to receive light andfocus the light to the front side waveguide.

In an aspect of the invention, a structure comprises a lens structure ona first side of a wafer. The lens structure comprises an oxide materialwith a plurality of trenches of a predefined pattern filled with amaterial with a different index of refraction as the oxide material. Thestructure comprises a coupler provided on a second side of the wafer andaligned with light provided from the lens; and a waveguide coupled tothe coupler.

In an aspect of the invention, a method comprises providing adiffractive lens on a backside of a wafer; and connecting a waveguide toa front side of the wafer, wherein the diffractive lens configured toreceive light and focus the light to the waveguide connected to thefront side of the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows an overview of an example wafer with a backside lens inaccordance with aspects of the present invention.

FIGS. 2 and 3 show manufacturing processes and respective structures inaccordance with aspects of the present invention.

FIG. 4A shows an example wafer with a backside lens in accordance withaspects of the present invention.

FIG. 4B shows a top down view of the adiabatic coupler in accordancewith aspect of the invention.

FIG. 5 shows an example wafer with a backside lens in accordance withaspects of the present invention.

FIG. 6 shows an exemplary pattern of a binary diffractive lens inaccordance with aspects of the present invention.

FIG. 7 shows another exemplary pattern of a binary diffractive lens inaccordance with aspects of the present invention.

DETAILED DESCRIPTION

The invention relates to semiconductor structures and, moreparticularly, to an integrated circuit with a backside binary gratedlens and a method of manufacture. More specifically, the presentinvention relates to an integrated circuit with a lens on a backside ofa substrate or wafer to focus a relatively large diameter light beam toa relatively small diameter waveguide provided on a topside of thesubstrate.

In accordance with aspects of the present invention, a lens is built ona backside of a substrate, e.g., a wafer of an integrated circuit, tofocus a beam of light from a fiber to a waveguide that has asubstantially smaller diameter than the beam of light, e.g., 30 to 200times smaller diameter. In embodiments, the lens may be a grated binarydiffractive lens that takes advantage of the thickness of the wafer tofocus the light from the fiber to the waveguide without substantialchanges in the direction of the light. Practically, binary opticalgrating design can be performed using computer aided simulations as isknown to those of skill in the art. As a result of the design of thepresent invention, the waveguide may receive the light, and since thelight has been focused to the waveguide with high efficiency, opticalloss is minimized. Also, light from a single fiber and having multipledifferent wavelengths, e.g., in wavelength division multiplexing (WDM)systems, can also be focused to the waveguide with minimized opticalloss.

In accordance with aspects of the present invention, a backside binarygrated lens is provided and coupled to a topside silicon waveguidethrough a vertical to horizontal coupler structure. In embodiments, thevertical to horizontal coupling may be a polysilicon spacer to ahorizontal single crystal silicon waveguide. In embodiments, the heightof the coupler may match the height of the waveguide. In embodiments,the polysilicon spacer can be about 0.1 micrometers to about 3micrometers thick, and preferably about 1 micrometer thick. Inembodiments, the binary grated lens may be a sequence of etched trenchesin the backside of the wafer and filled with nitride, oxide, and/orother materials.

The integrated circuit in accordance with aspects of the presentinvention can be fabricated using conventional fabrication processes.For example, the integrated circuit of the present invention can bemanufactured in a number of ways using a number of different tools. Ingeneral, though, the methodologies and tools are used to form structureswith dimensions in the micrometer and nanometer scale. Themethodologies, i.e., technologies, employed to manufacture the organicprobe substrate of the present invention have been adopted fromintegrated circuit (IC) and printed circuit board technology. Forexample, the structures of the present invention are realized in filmsof material patterned by photolithographic processes. In particular, thefabrication of the integrated circuit of the present invention usesthree basic building blocks: (i) deposition of thin films of material ona substrate, (ii) applying a patterned mask on top of the films byphotolithographic imaging, and (iii) etching the films selectively tothe mask.

FIG. 1 shows an example wafer with a backside lens in accordance withaspects of the present invention. As shown in FIG. 1, an integratedcircuit 100 may include a wafer 105 with a waveguide 125 provided on atopside of the wafer 105. A coupler 130 may connect the waveguide 125 tothe wafer 105. The wafer 105 includes a lens 110 integrated on anopposite side of the wafer 105, e.g., a bottom side of the wafer 105with respect to the waveguide 125 and the coupler 130.

In embodiments, the lens 110 may be a binary diffractive grated lens. Inoperation, the lens 110 receives light 120 from an optical fiber 115,e.g., from a bottom side of the wafer 105, and focuses the light 120 tothe waveguide 125, e.g., via the coupler 130. For example, when thelight 120 contacts the lens 110, the direction of the light 120 changestowards the coupler 130. In this way, the lens 110 reduces a width ofthe light 120 to converge to a smaller width. For example, the lens 110may focus the light 120 to approximately the width of the coupler 130.In embodiments, the lens 110 will focus the light 120 by a factor ofapproximately 200. For example, the lens 110 may focus the light 120from approximately 100 micrometers off center to approximately 0.5micrometers. As shown in FIG. 1, wafer 100 has a thickness T that isrelatively large, thus reducing the angle at which the light 120 changesdirection, and hence, minimizing optical loss.

FIGS. 2 and 3 show manufacturing processes and respective structures inaccordance with aspects of the present invention. As shown in FIG. 2,the wafer 105 may be a silicon on insulator (SOI) substrate with asilicon layer 225 formed on a buried insulator layer 220. The siliconlayer 225 can be any semiconductor layer composed of any suitablematerial including, but not limited to, Si, SiGe, SiGeC, SiC, Ge alloys,GaAs, InAs, InP, and other III/V or II/VI compound semiconductors wherethe semiconductors are transparent to light of the wavelengths that theapplication is using. The buried insulator layer 220 can be a buriedoxide material. The buried insulator layer 220 can be formed on a wafer,e.g., silicon 215. In embodiments, the SOI wafer 105 can be formed usingconventional processes, such as Separation by Implantation of Oxygen(SIMOX) or other bonding techniques.

The lens 110 can be formed onto the wafer 105, e.g., by depositing aninsulator layer 205 on the wafer 215. In embodiments, the insulatorlayer 205 is preferably an oxide material due to the index of refractionof oxide being substantially lower than the index of refraction ofsilicon. In this way, it becomes easy to alter the direction of incominglight, e.g., to a focal point, such as an opening of a waveguide or awaveguide coupler, such as a coupler 130 of FIG. 1.

In embodiments, the insulator layer 205 may be deposited using atomiclayer deposition (ALD), chemical vapor deposition, plasma-enhancedchemical vapor deposition (PECVD), and/or other conventional depositionprocesses. Following deposition of the insulator layer 205, the surfaceof the insulator layer 205 can be planarized using etch back techniques,e.g., chemical mechanical polish (CMP) techniques, and/or otherconventional planarization techniques. A photoresist is then applied tothe insulator layer 205. The photoresist is then exposed to energy(e.g., light) to form a pattern (openings). Trenches 210 are then etchedinto the insulator layer 205 through the openings of the photoresist.The photoresist is then removed using conventional strippants, e.g.,oxygen ashing.

As shown in FIG. 3, the trenches 210 are then filled with siliconnitride or other insulator material different than the insulator layer205 to form segments 230. The silicon nitride can then be planarized,thereby completing the construction of the lens 110 on the backside ofwafer 105. In embodiments, the insulator layer 205 and the segments 230together form a binary diffractive grated lens having a diffractiongrating that redirects light that contacts the lens 110. In embodiments,the distance between the segments 230, e.g., a grating distance, maydiffer than what is shown in FIG. 3. For example, the grating distancemay be based on the diffraction angle, or the angle in which thedirection of incoming light needs to be altered to focus the incominglight to a waveguide. As described herein, the grating distance may varyacross the lens 110, and may not necessarily be equal as shown in FIG.3. While the lens 110 is shown as a binary diffractive grated lensformed with silicon nitride material, in practice, other types of lensesmay be used to focus light from a backside of a wafer to a waveguideconnected to a front side of the wafer.

FIG. 4A shows an example backside lens in accordance with aspects of thepresent invention. As shown in FIG. 4A, a coupler 130 is provided on atopside of a wafer 100 in contact with a silicon 215 and an oxide layer310. In embodiments, the coupler 130 may include polysilicon material315, covered with an oxide material 305, 310. The oxide material 310 isformed on the silicon layer 225. The lens 110 is provided on an oppositeside of the wafer 105, e.g., on a bottom side of the wafer 105 oppositeof the coupler 130. The lens 110 is constructed to alter the directionof light 120 to coupler 130, e.g., focus the light 120 from a fiberoptic into the coupler 130. In embodiments, the angle of the light 120can vary depending on its portion contacting the lens, with respect tothe directions inversely proportional to the thickness T of the wafer105. Since the wafer 105 has a substantially large thickness (thedistance between when light contacts the lens 110 and the coupler 130),the angle that the light 120 changes directions is minimized and thenumber of refractive index changes in the path is minimized, therebyminimizing optical loss. In embodiments, the coupler 130 may beintegrated with a waveguide, such as the waveguide 125 shown in FIG. 1.

FIG. 4B shows a topside view of the polysilicon material 315 of thecoupler 130 coupling to the waveguide silicon layer 225 in accordancewith aspects of the invention. Both the polysilicon material 315 and thewaveguide silicon layer 225 have a portion of their length graded fromthin to thick (or thick to thin) to aid in adiabatic coupling reducingunwanted reflections at an abrupt material change.

FIG. 5 shows another example wafer with a backside lens in accordancewith aspects of the present invention. As shown in FIG. 5, a waveguide125 is coupled to the coupler 130. The waveguide 125 can be formed onthe wafer using conventional deposition lithography and etchingprocesses. The waveguide 125 can be a single crystalline siliconmaterial 405. As shown, the coupler 130 receives light 120 focused bythe lens 110, and the light 105 entering the coupler 130 is provided tothe waveguide 125. In embodiments, the coupler 130 and the waveguide 125may be integrated. That is, the coupler 130 may be a waveguide thatincludes the single crystalline silicon material 405.

FIG. 6 shows a cross-sectional view of a pattern of an example binarydiffractive lens in accordance with aspects of the present invention. Asshown in FIG. 6, the width of each segment 230 and the distance betweenthe segments 230 can vary across the lens 110. The width of eachsegments 230 and the distance between segments 230 is designed withconsideration to the required angle of refraction to focus incominglight at each point on the lens 110 to a focal point, e.g., a waveguide.In a middle of the lens 110, the width and distance is such that theangle of refraction for incoming light is minimal, e.g., assuming thefocal point is at a middle of the lens 110. For example, a larger angleof refraction for incoming light is provided as the light enters thelens 110 further away from the center.

FIG. 7 shows another example of a binary diffractive lens in accordancewith aspects of the present invention. As shown in FIG. 7, segments 230may be arranged as concentric circles when viewed from the top. Thearrangement of the segments 230 cause a beam of light within a fiberwith a diameter of approximately 100 micrometers to focus to an openingof a waveguide having approximately a 0.5 micrometer diameter.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A method comprising: connecting a waveguide to afront side of a wafer; forming an insulator layer on a backside of thewafer; etching a plurality of trenches through the insulator layer; andforming segments by filling the plurality of trenches with material,wherein each of the segments extends from a first surface of theinsulator layer to a second surface of the insulator layer, wherein theinsulator layer and the segments constitute a diffractive lensconfigured to receive light and focus the light to the waveguideconnected to the front side of the wafer.
 2. The method of claim 1,wherein the material has a substantially different index of refractionthan the insulator layer
 3. The method of claim 1, wherein the pluralityof trenches are filled with a silicon nitride material.
 4. The method ofclaim 1, further comprising providing a coupler including a polysiliconmaterial and connecting the waveguide to the wafer, wherein the couplerreceives light redirected by the insulator layer and the material filledin the trenches.
 5. The method of claim 4, wherein the coupler includesa polysilicon material.
 6. The method of claim 1, wherein the light isreceived by the lens from an optical fiber.
 7. The method of claim 1,wherein the diffractive lens is a binary diffractive lens.
 8. The methodof claim 1, wherein the trenches that each extend from the first surfaceof the insulator layer to the second surface of the insulator layer, thesecond surface of the insulator layer contacting the backside of thewafer.
 9. The method of claim 8, wherein each of the segments comprisesan insulator material with a surface that is co-planar with the firstsurface of the insulator layer.
 10. The method of claim 1, wherein eachof the segments comprises an insulator material with a surface that isco-planar with the first surface of the insulator layer.
 11. A structurecomprising: a wafer; a waveguide on a front side of the wafer; a coupleron the front side of the wafer; a diffractive lens on a backside of thewafer opposite the front side of the wafer; wherein the diffractive lensreceives light in a first direction and focuses the light toward thecoupler; and the coupler transmits the light to the waveguide in asecond direction different than the first direction.
 12. The structureof claim 11, wherein the coupler has a 90-degree elbow shape.
 13. Thestructure of claim 11, wherein the diffractive lens is a binarydiffractive lens.